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A small analog model — the SRF-39FP — has a clear case that prevents inmates from using it to smuggle goods. And it only costs $30, which makes it the most inexpensive radio available in prison, according to The New Yorker.

The best part: The radio can play for 40 hours off of one AA battery. That's longer than an iPod Classic can play on a full charge.

Battery life is especially important for inmates, who have a limited spending budget per month.

A Sony spokesperson confirmed for The New Yorker that selling radios in American prisons creates sizable sales for the company, and even though Sony first released the SRF-39FP model 15 years ago, it's still popular behind bars.

Partially, perhaps, thanks to prison culture, according to The New Yorker: When inmates get released, they usually leave their radios behind, seeing it as either bad luck or a break of "convict code" to bring them along with them in the free world.